Locating devices used in precision machines typically have two kinds: static locating devices and dynamic locating devices.
A typical static locating device generally includes a photoelectric switch or a limit switch for locating position of the precision machine. The precision machine has a movable stage. When the movable stage touches the photoelectric switch or the limit switch, the required position of the precision machine is located. However, this kind of static locating device cannot be used for real time controlling, thus it cannot be used for locating a plurality of positions.
The dynamic locating device has an optical linear scale and a servo controller such as a computerized numerical control (CNC) device. When combined with the servo controller, a position sensor can locate the position of the precision machine. The optical linear scale is a linear optical encoders which is made according to the optical diffraction theory. A typical optical linear scale includes a main scale, an index scale, a photoelectric conversion part, and an absolute interpolation means. The main scale is provided with a first cut line showing at least an original position and a second cut line graduated longitudinally at equal intervals. The index scale is provided with a cut line across said first and second cut lines, which is placed so as to be movable relative to said main scale. The photoelectric conversion part is for detecting Moire fringes generated by the cut lines between said two scales and for generating a signal having sinusoidal waveform and an original detecting signal showing a range of the above-mentioned original positions. The signal having sinusoidal waveform changes by one period every relative movement by unit length. The absolute interpolation means is used for outputting an inside value of the above-mentioned unit length by generating a pulse signal according to a prescribed phase deviation of the above-mentioned signal having sinusoidal waveform. An original position signal is generated by a logical product of an interpolated count value showing specific interpolation position information outputted from the above-mentioned absolute interpolation means and the origin detecting signal showing the above-mentioned origin position. When the optical linear scale is used for locating position of a precision machine, it must be disposed on the precision machine, thus, the optical linear scale is prone to be contaminated by a cutting liquid or even be damaged by an mishap in manufacture. Thus, the locating accuracy for the precision machine is decreased.
What is needed, therefore, an optical locating device for use in a precision machine which can be used for real time control, and can achieve high locating accuracy.